A. Field of the Disclosure
The field of this disclosure is advanced anti-corrosive coatings for metal surfaces.
B. Background of the Art
Corrosion is an electrochemical process in which a current leaves a structure at an anode site, passes through an electrolyte, and reenters the structure at a cathode site. For example, one small section of a pipeline may be anodic because it is embedded in soil that is more conductive than the soil along the rest of the pipeline. Current flows between the anode and cathode facilitated by the conductive pipe and the electrolytic soil or groundwater medium. In the process, the metal in the pipe undergoes a shift in oxidation state that results in electrochemical corrosion.
Cathodic protection is a method to reduce corrosion by minimizing the difference in potential between anode and cathode. This is achieved by applying a current to the structure to be protected from some outside source. When enough current is applied, the whole structure will be at one potential; thus, anode and cathode sites will not exist. Cathodic protection is commonly used on many types of structures in corrosive electrolytic environments, such as pipelines, underground storage tanks, locks, and ship hulls.
There are two main types of cathodic protection systems: galvanic and impressed current. A galvanic cathodic protection system makes use of the corrosive potentials for different metals. Without cathodic protection, when one area of the structure has a higher negative potential than another, corrosion results. If, however, a much less inert object (that is, an object with much more negative potential) is placed adjacent to the structure to be protected, such as a pipeline, and an electrical connection exists between the object and the structure, the object will become the anode and the entire structure will become the cathode. The new object corrodes instead of the structure thereby protecting the structure. In this example, the object is called a “sacrificial anode.” Thus, the galvanic cathodic protection system is also called a “sacrificial cathodic protection system” because the anode corrodes “sacrificially” to protect the structure. Galvanic anodes are made of metals with higher negative potential than the metal of the structure itself; the metal of the anode is said to be “anodic” compared to the metal of the structure.
Attachment of the anode is normally done at the jobsite utilizing underground insulated wire, thermite weld, conventional weld, or threaded bolts. Problems related to these methods of attachment to the structure include, but are not limited to: improper placement of the anode, improper size of the anode, improper composition of the anode, damage to the metallic object or the internal lining from excessive heat from the weld, loss of structural integrity of the metallic object, damage to the anode, wire, or electrical connection during installation and backfilling operations, improper weld or connection at the jobsite resulting in the loss or reduction in effectiveness of protection, and failure to remove protective wrapping from the anode prior to burial. Traditional sacrificial anode placement also requires extra trench excavation either several feet below the structure or several feet to the side of the structure. As a result, there is a long-felt need in the art for an effective means of galvanic protection without these limitations.
One method of protection which solves the problems listed above is a metallic sacrificial coating applied directly to the structure's surface. The coating acts as a barrier between the metal in the structure and the environment, and if breached it acts as a sacrificial anode to prevent corrosion. However, traditional methods of providing a sacrificial coating have several drawbacks. Traditional methods, such as hot-dip galvanizing, electroplating, thermal diffusion galvanizing, and vapor galvanizing are expensive, energy-intensive, time-intensive, and provide poor control of the thickness and consistency of the anodic coating. Accordingly, there is a long-felt need in the art for methods of applying corrosion protection that allow rapid, inexpensive, thin yet complete coatings to be applied.
As materials become increasingly expensive, there is a need to provide thin yet effective galvanizing layers. Although thin layers of anodic metal have the advantage of lower weight and cost, they have the disadvantages of being easily damaged or worn off. An anodic layer can be protected by a barrier layer, but over time barrier layers have a tendency to delaminate and allow corrosive material to contact the anodic layer. Accordingly, there is a long-felt but unmet need in the art for a galvanically protective coating paired with a barrier coating that will be durable, long-lasting, and with superior adhesion characteristics.